Researchers use a fiber optic cable to look at a single molecule.

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When I think of spectroscopy, I get two images; one is of white coated chemists high on ether fumes, ramming cuvettes of material through a machine that prints out pretty spectra—like CSI with better dialog. The second image is of bespeckled physicists and engineers working late at night with ultra-stable lasers and weird electronic equipment, trying to sense molecules at concentrations down to less than one part per trillion. However, it turns out that these images don't have to be true. Evidently chemists use fume cupboards and, in principle, a single atom or molecule can absorb all the light you throw at it.

When trying to figure out how many photons a substance can absorb, physicists think in terms of a cross-section. The idea is that atoms and molecules are replaced with an area that depends on many different parameters of the material, and every photon that passes through that area is absorbed. The problem with doing this experiment is that the cross-section is so small that special optics are required to focus the laser beam to the required area. There are two methods available to focus the laser beam sufficiently. One, using a lens with a numerical aperture greater than one—such as those used in a modern lithography plant. However, these would require that the atom to be looked at sits in fluid, which makes things complicated. Alternatively, the light can be sent down a fiber optic cable that is tapered until the internal core, where the light is confined, is smaller than the wavelength of the light. At this point the light ceases to propagate in the fiber and instead travels along an aluminum coating by exciting the electrons to move coherently. When the light exits the fiber it has the same physical dimensions as the fiber core, and is very intense. It also rapidly expands, so anything that changes the intensity of the collected light must have occurred within the tiny volume of space where the light was tightly focused.

Now that the light can be focused as it needs to be, the atom or molecule must be found. The problem is that gas molecules move around rather fast and tracking them with a fiber optic cable is quite difficult. To get around this, the researchers coated a glass slide with a molecule that almost behaved like an ideal atom. The coating was thin enough that they could be certain that the probe would only illuminate one molecule. Once the researchers located a molecule and positioned the probe correctly, the molecule absorbed about 6% of the light.

This isn't quite "all the light" as promised by quantum mechanics but then the molecule the researchers used isn't the idealized atom that the calculations are based on. Compared to high sensitivity gas spectroscopy measurements, where the light travels through kilometers of gas to detect at concentrations of one part per trillion, 6% absorption by a single molecule is incredibly strong. The researchers also point out that the light emitted by the molecules, which was used to align the probe, showed that they could easily detect very weak photon emitters. This may be important in imaging because many techniques rely on staining materials with florescent molecules, and more sensitive detection may mean more accurate imaging.

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Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He Lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com